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<html xmlns="http://www.w3.org/1999/xhtml">

<head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    <title>判断数字是否为两位数</title>
    <style>
        body {
            font: 12px/1.5 arial;
            text-align: center;
        }

        .f-text {
            width: 50px;
            border: 1px solid #ccc;
            background: #f0f0f0;
            font-family: inherit;
            padding: 3px;
            margin-right: 10px;
        }
    </style>
    <script type="text/javascript">
        window.onload = function () {
            let aInput = document.querySelectorAll("input");
            let aSpan = document.querySelector("span");
            let i = 0;

            aInput[0].onkeyup = function () {
                this.value = this.value.replace(/[^\d]/g, "")
            }

            aInput[1].onclick = function () {
                !aInput[0].value ? alert('请输入数字') : alert(/^d{2}$/.test(parseInt(aInput[0].value)) ? '√ 是两位数' :
                    '这是' + aInput[0].value.length + '位数')
            }
        };
    </script>
</head>

<body>
    <input type="text" class="f-text" /><input type="button" value="是否为两位数" />
</body>

</html>